U.S. Secretary of State Hillary Clinton has dismissed Iran's offer to resolve international concerns about its nuclear program if the West starts lifting sanctions, saying the "burden of action" is on Tehran to make the first move.

Speaking Monday on a visit to Brasilia, Clinton said Iran must demonstrate "seriousness" in upcoming nuclear negotiations with six world powers, due to be held in Iraq in late May. She said the United States will keep sanctions in place and maintain pressure on Iran as it considers what to offer at the talks and will "respond accordingly" to any Iranian proposal.

Earlier, Iranian Foreign Minister Ali Akbar Salehi told Iran's ISNA news agency that his government always will assert its right to enrich uranium, but also will be open to negotiations about the degree of enrichment that it pursues. Western powers suspect Iran is trying to enrich uranium to a weapons-grade level to enable it to assemble an atomic bomb. Iran says its nuclear program is only for peaceful purposes.

Salehi urged Western nations to begin easing sanctions against Iran if they want to resolve the dispute at the next round of nuclear talks, which will follow a meeting held by all the parties in Istanbul last Saturday. He welcomed those talks with diplomats from the five permanent members of the United Nations Security Council (the United States, Britain, China, France and Russia) plus Germany as constructive.

Meanwhile, Reuters news agency reports Denmark's foreign minister as saying there is no room for easing sanctions on Iran until it takes steps to comply with demands on its nuclear program. Denmark holds the European Union's rotating presidency.

On Sunday, U.S. President Barack Obama insisted that his administration did not "give away anything" to Iranian negotiators in Istanbul. Speaking to reporters in Cartagena, Colombia, where he attended the Summit of the Americas, he defended Washington's decision to continue to push for a diplomatic resolution of the dispute about Tehran's nuclear ambitions.

Mr. Obama said that while he refused to let the negotiations turn into a "stalling process," he was willing to give diplomacy one last chance. The president was responding to remarks by Israeli Prime Minister Benjamin Netanyahu, who said earlier Sunday that the United States and world powers gave Tehran a "freebie" by agreeing to hold another round of talks next month in Baghdad.

International pressure on Iran has been increasing. New U.S. and European Union economic sanctions are due to go into effect July 1, while Israel has warned it may take military action to stop what it believes is a secret Iranian nuclear weapons program bent on destroying the Jewish state.